In many applications, from the fabrication of solid-state chemical sensors to preparation of biomedical test plates, it is important to be able to dispense a known quantity of liquid onto a solid surface, and to have it confined to desired lateral dimensions. A good example of this is the deposition of polymeric membrane solutions for potentiometric liquid chemical sensors. The size (and therefore cost) of these sensors is usually determined by the membrane dimensions and spacing.
The size of integrated ion sensors is dictated by the size and spacing of their polymeric membranes, rather than by the size of the associated circuitry. Polymeric membranes have been developed for automated deposition by screen printing, as shown by R. W. Hower et al., “New Solvent System for the Improved Electrochemical Performance of Screen-Printed Polyurethane Membrane-Based Solid-State Sensors”, PROCEEDINGS FOR TRANSDUCERS 95/EUROSENSORS IX, June 1995, pp. 858-862.
Such automated deposition can also be done with dispensing equipment as shown by S. Anna et al., “An IC-Technology Compatible Automatic Method (SCZ Method) for Immobilization Membranes”, SENSORS AND ACTUATORS, vol. B1, pp. 514-517, 1990.
In both cases, membrane components are dissolved in solvents which are evaporated subsequent to deposition. The area occupied by an array of these membranes can be significantly reduced through the use of wells, areas separated by barrier walls, into which the membrane solutions are deposited. Thick wells for screen-printed membranes are shown by the above-noted article by Hower et al.
Membrane design rules are typically dictated by the requirement of keeping membranes which are selective to different chemicals from touching. If these membranes touch, their ionophores-intermix, causing cross-contamination. As mentioned above, membranes can be deposited automatically by either screen-printing or dispensing equipment. The membrane components are dissolved in solvents to form a paste for screen-printing or a liquid for dispensing. Membrane design rules have needed to allow for flow-out of the paste or dispensing solution after it is applied to the sensor surface, making the sensors much larger than they would otherwise need to be.
To reduce the size of screen-printed sensor arrays, wells, as illustrated in FIGS. 1a and 1b, have been formed which limit the flow-out of the membrane components, allowing membranes to be smaller and closer together, as further shown in the above-noted article by Hower et al. These wells provide the additional advantage of making final membrane thickness more uniform and the deposition process more tolerant of variations in the viscosity of membrane solutions.
Epoxies, acrylic photo polymers, thick film polyimide, and silicon have been used to form wells or cavities, as further shown in U.S. Pat. No. 5,200,051 issued to Cozzette et al., and the articles by L. J. Bousse et al., “Silicon Micromachining in the Fabrication of Biosensors Using Living Cells”, TECHNICAL DIGEST, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., p. 173-6; June 1990; and R. Eugster et al., “Selectivity-Modifying Influence of Anionic Sites in Neutral-Carrier-Based Membrane Electrodes”, ANALYTICAL CHEMISTRY, vol. 63, pp. 2285-2289 (1991).
The approaches previously described are quite acceptable for screen-printed silicone and polyurethane membranes, as they are viscous, thixotropic pastes.
Several epoxies have excellent chemical compatibility with the membranes as well as good membrane adhesion and screen-printing properties, as shown by the article by R. W. Hower et al., “Study of Screen-Printed Epoxies for Wells in Solid-State Ion Selective Electrodes”, TECHNICAL DIGEST, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C., 1996. Membrane solutions optimized for dispensing, on the other hand, have low viscosities and a high solvent-to-solids ratio to keep the dispensing tip from clogging; the composition of a typical membrane is over 90% solvent. When these low-viscosity membrane cocktails are dispensed into the thick wells, the membranes wick out of the wells through surface tension, thinning the resulting membranes and enlarging the required membrane area.
Microsensors are sensors that are manufactured using integrated circuit fabrication technologies and/or micromachining. Integrated circuits are fabricated using a series of process steps which are done in batch fashion, meaning that thousands of circuits are processed together at the same time in the same way. The patterns which define the components of the circuit are photolithographically transferred from a template to a semiconducting substrate using a photosensitive organic coating. The coating pattern is then transferred into the substrate or into a solid-state thin-film coating through an etching or deposition process. Each template, called a “mask”, can contain thousands of identical sets of patterns, with each set representing a circuit. This “batch” method of manufacturing is what makes integrated circuits so reproducible and inexpensive. In addition, photoreduction enables one to make extremely small features. The resulting integrated circuit is contained in only the top ¼ micron or so of the semiconductor substrate and the submicron thin films on its surface. Hence, integrated circuit technology is said to consist of a set of planar, microfabrication processes.
Micromachining refers to the set of processes which produce three-dimensional microstructures using the same photolithographic techniques and batch processing as for integrated circuits. Here, the third dimension refers to the height above the substrate of the deposited layer or the depth into the substrate of an etched structure. Micromachining produces third dimensions in the range of 1-500 μm (typically). The use of microfabrication to manufacture sensors produces the same benefits as it does for circuits: low cost per sensor, small size, and highly reproducible behavior. It also enables the integration of signal conditioning, compensation circuits and actuators, i.e., entire sensing and control systems, which can dramatically improve sensor performance for very little increase in cost. For these reasons, there is a great deal of research and development activity in microsensors.